Conventional MEMs mirrors for use in optical switches, such as the one disclosed in U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 to Buzzetta et al, redirect beams of light to one of a plurality of output ports, and include an electro-statically controlled mirror pivotable about a single axis. Tilting MEMs mirrors, such as the ones disclosed in U.S. Pat. No 6,491,404 issued Dec. 10, 2002 in the name of Edward Hill, and United States Patent Publication No. 2003/0052569, published Mar. 20, 2003 in the name of Dhuler et al, which are incorporated herein by reference, comprise a mirror pivotable about a central longitudinal axis. The MEMs mirror device 1, disclosed in the aforementioned Hill patent, is illustrated in FIG. 1, and includes a rectangular planar surface 2 pivotally mounted by torsional hinges 4 and 5 to anchor posts 7 and 8, respectively, above a substrate 9. The torsional hinges may take the form of serpentine hinges, which are disclosed in U.S. Pat. No. 6,327,855 issued Dec. 11, 2001 in the name of Hill et al, and in United States Patent Publication No. 2002/0126455 published Sep. 12, 2002 in the name of Robert Wood, which are incorporated herein by reference.
One of the main challenges facing MEMS designers of larger sized mirrored platforms, e.g. 2 mm to 3 mm in length, is the conflicting requirement of high mirror resonance frequency and low stress-induced mirror curvature. The former demands a relatively thin light mirror, while the latter requires a relatively thick structure. When the mirror is too thin, the reflective surfaces will have excessive curvature induced by the stresses in the reflective coatings or internal stresses in the mirror itself, which results in excessive optical coupling losses. However, making the mirror too thick makes it heavy, thereby lowering the resonant frequency for a given hinge stiffness. Moreover, increasing the hinge stiffness to compensate for a heavy mirror would require too high a voltage to drive the mirror electro-statically to the desired angle.
U.S. Pat. No. 6,791,730 issued Sep. 14, 2004 to Sniegowski et al discloses a micro-mirror structure including stiffening ribs or rails between upper and lower layers. The structure disclosed in the Sniegowski et al reference is realized using surface micro-machining processes, which are generally limited to the manufacture of relatively small mirrors. Many optical switching applications require large area mirrors tilting to a relatively high angle, thereby requiring a large swing space underneath, which is difficult to achieve using surface micro-machining. Moreover, when closed cells are used as the stiffening members, access holes have to be etched on the optically active upper layer to allow for the removal of any sacrificial layers that are disposed therebetween, resulting in a plurality of dimples or the like formed in an upper surface of the upper layer, which have an adverse impact on the optical performance capabilities thereof, thereby making the mirror unacceptable in many applications.
An object of the present invention is to overcome the shortcomings of the prior art by providing a sandwich structure including upper and lower smooth and solid skins, and a closed cellular core to minimize curvature and maximize resonance frequency.